Digital System Design-II (CSEB312) Verilog Tutorial Part - 2

Topics Sequential Logic More Combinational Logic Finite State Machines

Sequential Logic Verilog uses certain idioms to synthesize into latches, flip-flops and FSMs Other coding styles may simulate correctly but produce incorrect hardware

Always Statement General Structure: always @ (sensitivity list) statement; Whenever the event in the sensitivity list occurs, the statement is executed

D Flip-Flop module flop(clk, d, q); input clk; input [3:0] d; output [3:0] q; reg [3:0] q; always @ (posedge clk) begin q <= d; // read as “q gets d” end endmodule Any output assigned in an always statement must be declared reg. In this case q is declared as reg Beware: A variable declared reg is not necessarily a registered output. We will show examples of this later.

Resettable D Flip-Flop module flopr(input clk, input reset, input [3:0] d, output reg [3:0] q); // synchronous reset always @ (posedge clk) begin if (reset) begin q <= 4'b0; end else begin q <= d; endmodule

Resettable D Flip-Flop module flopr(input clk, input reset, input [3:0] d, output reg [3:0] q); // asynchronous reset always @ (posedge clk, posedge reset) begin if (reset) q <= 4'b0; else q <= d; end endmodule

D Flip-Flop with Enable module dff_en(input clk, input reset, input en, input [3:0] d, output reg [3:0] q); // asynchronous reset and enable always @ (posedge clk, posedge reset) if (reset) q <= 4'b0; else if (en) q <= d; endmodule

Latch module latch(input clk, input [3:0] d, output reg [3:0] q); always @ (clk, d) if (clk) q <= d; endmodule Warning: We won’t use latches in this course, but you might write code that inadvertently implies a latch.

Other Behavioral Statements Statements that must be inside always statements: if / else case, casez Reminder: Variables assigned in an always statement must be declared as reg (even if they’re not actually registered!)

Combinational Logic using always // combinational logic using an always statement module gates(input [3:0] a, b, output reg [3:0] y1, y2, y3, y4, y5); always @ (*) begin // need begin/end because there is // more than one statement in always y1 = a & b; // AND y2 = a | b; // OR y3 = a ^ b; // XOR y4 = ~(a & b); // NAND y5 = ~(a | b); // NOR end endmodule This hardware could be described with assign statements using fewer lines of code, so it’s better to use assign statements in this case.

Combinational Logic using case module sevenseg(input [3:0] data, output reg [6:0] segments); always @(*) case (data) // _abc_defg 0: segments = 7'b111_1110; 1: segments = 7'b011_0000; 2: segments = 7'b110_1101; 3: segments = 7'b111_1001; 4: segments = 7'b011_0011; 5: segments = 7'b101_1011; 6: segments = 7'b101_1111; 7: segments = 7'b111_0000; 8: segments = 7'b111_1111; 9: segments = 7'b111_1011; default: segments <= 7'b000_0000; // required endcase endmodule

Combinational Logic using case In order for a case statement to imply combinational logic, all possible input combinations must be described by the HDL. Remember to use a default statement when necessary.

Combinational Logic using casez module priority_casez(input [3:0] a, output reg [3:0] y); always @(*) begin casez(a) 4'b1???: y = 4'b1000; // ? = don’t care 4'b01??: y = 4'b0100; 4'b001?: y = 4'b0010; 4'b0001: y = 4'b0001; default: y = 4'b0000; endcase end endmodule

Blocked vs. Non-Blocked Assignments Blocked Assignment: One statement completes before next one completed. Statements are sequential so the statement order makes a difference. Use = operator Non-Blocked Assignment: Statements execute in parallel (so the order does not affect the outcome) Use <= operator

Blocked vs. Non-Blocked Assignments Example: Shift Register (ref p 159) – Using blocked assignment statements module shift_reg(E, A, rst, clk); output E; input A, rst, clk; reg A, B, C, D; always @ (posedge clk or posedge rst) begin if (rst) begin A=0; B=0; C=0; D=0; end else A = B; B = C; C = D; D = E; end # of if end // of always endmodule

Rules for Signal Assignment Use always @ (posedge clk) and nonblocking assignments to model synchronous sequential logic always @ (posedge clk) q <= d; // nonblocking Use continuous assignments to model simple combinational logic. assign y = a & b; Use always @ (*) and blocking assignments to model more complicated combinational logic where the always statement is helpful. Do not make assignments to the same signal in more than one always statement or continuous assignment statement.

Finite State Machines (FSMs) Finite state machines are Synchronous sequential circuits drawn in a form as shown below. The name FSM is used because k registers (sets of flip-flops) can have 2n states. An FSM has M inputs, N outputs, and k bits of states. FSM receives a clock and may be a reset also. On each clock edge, the FSM moves to the next state. Two types of FSMs are: Mealy and Moore.

Finite State Machines (FSMs) Besides registers, there are two combinational blocks in an FSM: Next state logic Output logic So in Verilog, we can three blocks of code next state logic state register output logic

FSM Example: Divide by 3 The double circle indicates the reset state

// ---- output logic ---- FSM in Verilog module divideby3FSM (input clk, input reset, output y); reg [1:0] state, nextstate; parameter S0 = 2'b00; parameter S1 = 2'b01; parameter S2 = 2'b10; // ---- state register ---- always @ (posedge clk, posedge reset) begin if (reset) state <= S0; else state <= nextstate; end end // always // ---- next state logic ---- always @ (*) begin case (state) S0: nextstate <= S1; S1: nextstate <= S2; S2: nextstate <= S0; default: nextstate <= S0; endcase end // always // ---- output logic ---- always (posedge clk) begin if (state = = S0) y = 1; else y = 0; endmodule